A method of forming a disc brake

Abstract: A disc brake assembly includes a disc (50) having an axis of rotation (51), a radial inner portion (54) and a radial outer portion (55) separated by a continuous circumferential gap (56). Brake pad assemblies (11, 12) are located on respective sides of the disc (50), each engagable with both of the radial inner and outer portions (54, 55). A piston cylinder assembly having a piston (26) and a cylinder (24) is provided for movement of the brake pad assemblies (11, 12) into engagement with the disc (50). The cylinder (24) is connected to the inboard pad assembly (11)and a draw bar (29), which extends co-axially with the axis of movement of the piston cylinder assembly through the gap (56) and connects the piston (26) to the outboard pad assembly (12).

Description: The invention relates to a method of forming a disc for a disc brake, and is particularly but not exclusively concerned with such brakes as used with motor vehicles. It will be convenient to hereinafter describe the invention by reference to motor vehicle wheel brakes, but it is to be understood that the invention has wider application. In a typical disc brake assembly for a motor vehicle, a caliper bridge is arranged to straddle a rotatable disc. The bridge incorporates inboard and outboard portions each of which incorporates an associated friction pad. The portions are able to move relative to one another to enable the friction pads to engage opposing faces of the disc under the influence of a hydraulic piston cylinder assembly acting on the inboard portion. In operation, stresses are induced in the brake assembly which cause deflection of the caliper. In one form of deflection, the caliper may stretch in an axial direction, i.e. in a direction parallel to the axis of rotation of the disc. In another form, the caliper may yawn or splay under load causing uneven loading to be applied to the disc by the pad assemblies. Furthermore, the caliper may tilt relative to the axis of the rotation of the disc causing the pads to tend to taper after continued use. These deflections are undesirable as they increase the fluid required to actuate the brakes, cause variations in the specific torque output of the brake assembly under different loadings, and result in uneven wearing of the pads. To compensate for the effects of this loading, different caliper designs have been proposed to minimise the deflection of the caliper. However, these designs have not been completely effective in stopping deflection of the caliper. Furthermore, these designs often add to the weight of the assembly, the complexity of its manufacture and the manufacturing costs. In addition these assemblies are often unable to dissipate heat as effectively due to the additional structure provided to make the brake assembly stiffer. GB-A-0654885 discloses a disc brake in which two concentric brake discs are provided with an annular gap therebetween wherein friction pads are provided on either side of both brake discs with a piston of the brake cylinder extending through the annular gap. DE-A-3815690 discloses a lathe capable of machining two opposite sides of a workpiece simultaneously. The present invention consists in a method of forming a disc for a disc brake assembly, the disc being rotatable about an axis of rotation and including a radially outer portion and a radially inner portion, each of the outer portion and the inner portion having two opposite sides thereof and a gap forming a radial separation between said outer and inner portion and extending in a continuous circle coaxial with the axis of rotation, the method characterised by the steps of: (i) providing a tool including a central shaft and including at least one cutting head having a cutting edge radially spaced from, and rotatable about, the shaft; (ii) locating the cutting tool such that the cutting head is on one side of the disc with the shaft being substantially aligned with said gap; and (iii) rotating the shaft to cause the cutting head to move in a circular path such that the cutting edge engages the one side of both the inner and outer portions of the disc on rotation of the shaft to thereby machine braking surfaces on the one side of both the inner and outer portions of the disc. The disc may be formed in one piece or may be formed in separate pieces with the aperture and the contact surfaces machined by any or all of grinding, turning and milling of the disc. It will be convenient to hereafter describe various embodiments of the invention in greater detail with reference to the accompanying drawings. The particularity of these drawings in the related description is not to be understood as superseding the generality of the preceding broad description of the invention. In the drawings: Figure 1 is an exploded perspective view of a sub assembly for a disc brake assembly for use with a disc made by the method in accordance with the invention; Figure 2 illustrates a variation of the sub assembly of Figure 1; Figure 3 is a sectional view of a first embodiment of a disc made by the method in accordance with the invention; Figure 4 is a sectional view of a second embodiment of a disc made by the method in accordance with the invention; Figure 5 is a variation of the disc of Figure 4; Figure 6 is a perspective view of a disc brake assembly including the sub assembly of Figure 1; Figure 7 is a sectional view along section line VII-VII of Figure 6 and illustrating a full cast disc; Figure 8 is a perspective view of a variation of the brake pad assembly for use in the sub assembly of Figure 1; Figure 9 illustrates a machining tool for preparation of the braking surfaces of the disc of the disc brake assembly; Figure 10 is a variation of the machining tool of Figure 9 incorporating two cutting heads; and Figure 11 is a schematic view of the braking surfaces on a disc using the machining tools of either Figure 9 or 10. Figure 1 illustrates a sub assembly 10 of a disc brake assembly 100. This figure is an exploded view to more clearly disclose the various component parts. The sub assembly incorporates inboard and outboard friction pad assemblies (11 and 12 respectively), each of which incorporates a support plate (13, 14) and a friction pad (15, 16). Typically, each friction pad is bonded or riveted to its respective support plate to transmit shear load. An anchor bracket 17 is adapted to be mounted to a motor vehicle through mounting holes 18. Guide rails 19 extend along opposing sides of the bracket to support the inboard and outboard assemblies. Cooperable slots 20 are located on the respective inboard and outboard assemblies 11, 12 and receive the guide rails enabling the assemblies to be slidably mounted on the anchor 17 in a direction away or towards each other. It should be appreciated that the pad assemblies may be secured to the anchor bracket 17 in other ways, such as with slideways located on the anchor bracket arranged to receive respective ends of the assemblies, or by a pin guided arrangement. Two cylinder piston assemblies (22, 23) form the actuating means 21 of the sub assembly 10. Each cylinder piston assembly includes a cylinder 24 mounted to the inboard pad assembly through bracket 25, and piston 26. In each cylinder piston assembly, the piston 26 is slideable within the cylinder 24 along a movement axis 71 (Figure 7) and movable in a direction away or towards the support plate 13 of the inboard assembly 11 under hydraulic pressure. A seal 27 (Figure 7) is located between the piston 26 and cylinder 24 and a boot (not shown) is located on the cylinder on the inboard side of the seal 27. It should be realised a single cylinder could be used and that the number of cylinders in the assembly 10 may vary depending on the size of the assembly as well as the specific torque output required. Furthermore, the cylinders may be secured to the support plate 13 in any suitable form including integrally forming the cylinders with the support plate. Furthermore, it may be desirable to have only a reduced contact area between the cylinder and the plate and this arrangement is illustrated in Figure 7. Force transmitting means 28 in associated with the actuating means 21 to provide at least part of an interconnection between the pad assemblies such that the inboard assembly responds to movement of the outboard assembly towards or away from the disc. In the first embodiment, the force transmitting means 28 is in the form of rods 29 associated with each cylinder piston assembly and which extend coaxial with the movement axis. In Figure 1 the rods are integrally formed with respective piston 26. In an alternative arrangement (not shown) each piston is connected to the rod through any appropriate coupling means such as a screw thread arrangement. Rods 29 pass through apertures (30, 31, 32) in the respective cylinders 24 and pad assemblies 11 and 12 and are secured to the outboard support plate 14. The rods 29 are slideable within the apertures 30 and 31 enabling the rods to be movable relative to the cylinders 24 and the inboard pad assembly 11. A seal 33 (Figure 7) is provided in the respective apertures 30 to seal between the respective rods 29 and the cylinders 24. While the rods 29 may be secured to the support plate 14 in any suitable way, in the illustrated arrangement of Figure 1, each rod incorporates a threaded stud 34 on its outer end which is arranged to receive a cooperable nut 35. Furthermore, the rod 29 is keyed to the support plate 14 to prevent relative rotation thereof during tightening of the nut 35 by a portion 36 located adjacent the stud 34 and having a rectangular or any other non-circular shape cross-section which is received in the suitably shaped aperture 32 in the outboard support plate. Furthermore, the rods 29 are positioned across the support plate 14 at a position to optimise the load distribution induced on the plate 14 by the rods 29. In this arrangement, as there are two rods, the friction pad 16 may be divided notionally into two regions with each rod connected at a centre of a respective one of the regions. To evenly distribute the load across the plate the size of each region would be proportional to the expected actuation load induced by each rod. In the illustrated case as each rod is expected to transmit half the actuation load, the size of these regions is approximately the same. In an alternative arrangement shown in Figure 2, each rod 29 incorporates an enlarged T-shaped head portion 37 and each aperture (31, 32) in the pad assemblies is elongated to accommodate entry of the respective head portions. The rods 29 are arranged to be secured to the outboard support plate 14, once the respective heads have passed through apertures 32, by rotating the rods through 90°. The heads are biased towards support plate 14 to minimise rattle by some form of spring clip (not shown). At the same time this clip also acts as an antirotational device for the head. In this arrangement, the rods are typically connected to the pistons via screw threads (not shown). The friction pad (15, 16) of each pad assembly (11, 12) provides a operative surface (38, 39) which in use is arranged to engage respective sides of a disc 50 (Figure 7). While the friction pad may be unitary, in the illustrated arrangement, the operative surface of the respective pad assemblies incorporates channels 40 and 41 which divide each operative surface into four discrete parts. One channel 40 extends longitudinally across each pad assembly and incorporates the respective apertures (31, 32) to receive respective rods 29. The other channel 41 extends substantially transverse to this channel 40. Other arrangements of channels may be provided if required. As illustrated in Figure 8, a quick release pad assembly 42 is provided comprising a support plate 43 and friction pad 44. Apertures 45 are formed through the plate and pad which are arranged to receive respective rods 29. Each aperture is in the form of a radial slot which extends preferably from the radially lower edge 46 of the pad assembly 42 to enable quick release of the pad assembly from the brake sub assembly. Figure 3 illustrates a first embodiment of the disc 50 which may be used in association with the sub assembly 10 to form the disc brake assembly 100. The disc 50 is arranged to rotate about an axis of rotation 51 and rotate with the motor vehicle. The disc has an inboard side 52 and an outboard side 53 and incorporates a radially inner portion 54 and a radially outer portion 55 which are separated by gap 56 extending in a continuous circle coaxial with the axis 51 of the disc. Each portion (54, 55) has a respective inboard and outboard braking surface (57, 58, 59, 60) located on opposite sides of the disc 50. These braking surfaces provide the engagement surfaces for the operative surfaces (38, 39) of the respective pad assemblies. A bracket 61 is located on the outboard side of the disc 50 and interconnects the inner and outer portions (54, 55). The bracket 61 is secured to the inner and outer portions and is spaced from the inner and outer portion outboard braking surfaces (58, 60) to provide a cavity 62 in which the outboard assembly 12 is located, enabling the operative surface 39 of the outboard assembly to engage the outboard braking surface (58, 60) of the inner and outer portions. A second embodiment of the disc 50 is illustrated in Figure 4. This disc is similar to the disc of Figure 3 and incorporates the radially inner and outer portions (54, 55) separated by the gap 56. However in this arrangement, the disc forms part of the wheel assembly 63. The wheel assembly includes a hub 64, a rim 65, and a plurality of spokes 66 interconnecting the hub and rim. In Figure 4, the inner and outer portions are bolted, or otherwise rigidly connected, to the wheel assembly 63. This arrangement is feasible for use where the wheel incurs little side loading (eg. in cornering) and therefore, is not subjected to bending of the rim relative to the hub. An example of such a situation would be with a motor cycle. A variation of this disc 50 is illustrated in Figure 5 which may be used in motor vehicles where cornering loads could deflect the rim relative to the hub. In this case, the inner portion 54 is fixed to the hub 64 and the outer portion is attached to the rim 65 or spoke 66 in a manner that allows limited movement in the axial direction such as through a spline 67 as illustrated, or through key/keyways or drives. In alternative arrangements (not shown), to maintain correct alignment of the inner and outer portions, the inner portion is movable relative to the hub in an axial direction, while the outer portion is fixed, or both the inner and outer portions are movable in the axial direction with the sub assembly being fixed to form a datum surface. A further embodiment of the disc is illustrated in Figure 7 which comprises a fully cast disc 50. This disc is arranged to be connected solely to the hub and therefore deflection of the rim relative to the hub does not cause misalignment of the inner portion relative to the outer portion. Figures 6 and 7 illustrate the disc brake assembly 100 incorporating the sub assembly 10 and disc 50. The inboard and outboard pad assemblies (11, 12) are supported on the anchor bracket 17 and are located on opposing sides of the disc 50 with the guide rails 19 of the bracket 17 extending through the gap 56. The outboard pad assembly 12 is located in the cavity 62. The disc 50 includes a series of passages 68 which are formed in the bracket 61 to provide access to the cavity 62 and also improve ventilation around the disc. The cylinders 24 are connected to the inboard assembly 11 and the rods 29 are connected to the respective pistons 26 and extend through the gap 56 and are connected to the outboard pad assembly 12. The movement axis 71 of each piston relative the respective cylinder is substantially parallel to the axis of rotation 51 of the disc 50 and each piston 26 is arranged to move away from the inboard assembly in operation of the brake along the movement axis 71 with the rods 29 acting as a draw bar which draws the outboard assembly towards the inboard assembly causing the operative surface 39 to be drawn into engagement with the outboard braking surface (58, 60) of the disc 50. As the inboard assembly 11 is movable on the anchor bracket 17, on continued movement of the pistons 26 away from the inboard assembly 11, the inboard pad assembly is caused to move on the guide rails 19 on the bracket 17 to move the operative surface 38 of the inboard assembly 11 into engagement with the inboard face (57, 59) of the disc 50. An advantage of the disc brake assembly 100 is that each rod 29 is much better able to accommodate the loads induced in operation of the assembly than prior art caliper bridge arrangements by virtue of the location of each rod relative to the operative surfaces and to the axis of movement of the respective pistons. In this arrangement, the loads induced at these operative surfaces are transferred substantially along each rod with little bending moment being applied with any drag load or torque induced at the respective pad assemblies being transferred to the anchor bracket through their point of contact on the guide rails 19. As a result, the brake assembly provides a much stiffer arrangement with deflection of the brake assembly due to operational loading being substantially reduced. As a result, the operative surface of the pad assemblies on the disc will not change under increased loading thus enabling the brake assembly to give a more consistent specific torque output under different loadings. Furthermore, with the stiffer construction, the pad assemblies may include larger surfaces and the piston cylinder arrangement is more efficient requiring a lower fluid displacement. To inhibit binding of the piston within the cylinder without compromising the efficiency of the assembly, a portion 70 of the cylinder wall between seals 33 and 27 is relieved. In this way, the possibility of binding of the piston 26 can be reduced without compromising the effectiveness of the sealing arrangement between the piston 26 of the cylinder 24 or rod 29. A neat fit is provided between the cylinder and the piston or rod on either side of respective seals 33, 27. This ensures that extrusion of the seals does not occur which would otherwise increase fluid displacement. While the rod 29 takes very little bending movement, any side loading on the rod reacts against cylinder 27 at the location adjacent the seals 27, 33 which provide a bearing surface for the piston or rod. Furthermore, as each rod is more efficient in accommodating the stresses induced in the assembly, and as a caliper bridge is no longer required, the brake assembly can be of lighter construction and manufacturing costs can be reduced. Furthermore, as it is not required to provide space for a caliper bridge on the radial extremity of the disc, a larger diameter disc is possible which provides a larger heat sink for the brake assembly, and a larger effective radius. In addition the brake assembly 10 is well equipped to effectively dissipate heat in operation of the assembly. In relation to the disc, with the radial separation there is more surface area in the disc and vents 69 formed between the braking surface are shorter. Furthermore, the longitudinal channel 40 in each pad assembly is arranged to be located over the gap 56, providing a gap which allows air flow around the contact surfaces. The transverse channels 41 further assist in encouraging the air flow. Furthermore, with this arrangement, there is no caliper bridge or other structure to shield heat dissipation. A further advantage is that the brake assembly may be more easily removed from the disc. As a caliper bridge is not required, once the rods are disconnected from the outboard pad assembly, the brake assembly may be withdrawn from the disc by being moved in the axial direction. Furthermore, the pad assemblies may be changed without removal of the wheel. For example, in the brake assembly shown in Figure 2, the rods are released from the outboard pad assembly by being rotated through 90°, and this can be effected from the outboard side of the disc through passages 68. The inboard pad assembly and piston cylinder arrangement is then withdrawn from the disc in the axial direction and the outboard pad assembly is removed through the spokes in the wheel provided an adequate gap exists between adjacent spokes. A similar operation can be undertaken to install the new pad assemblies. Further benefits can be realised in the disc brake assembly 100 by the design and method of manufacture of the disc 50. The disc 50 may be formed in one piece or may be cast in separate pieces and assembled later. In one embodiment, the bracket 61 forms part of a centre support 76 which is secured to an axle for rotation with the vehicle. Typically the centre support is made from steel and is fabricated or pressed or formed from a combination thereof. The inner and outer portions (54, 55) of the disc are cast or otherwise formed on to the bracket. Each portion may be formed as a single piece or may be formed from a plurality of interfitting segments These portions may be cast from any suitable material including aluminium, metal matrix composition or cast iron. Alternatively, the bracket and inner and outer portions may be cast from a single material and integrally formed together as shown in Figure 7. To prepare the necessary inboard and outboard braking surfaces (57, 58, 59, 60), the portions may be machined using a chip producing process including grinding, turning and milling. The surfaces usually need to be planar and have a suitable surface texture to expedite burnishing of the friction pads. Figure 9 illustrates a suitable machining tool located in the disc 50 to provide the outboard braking surfaces (58, 60). The machining tool 77 incorporates a head 78 mounted on a rotatable shaft 79. Cutters 80 (which may include single point or multiple point cutting edges) are secured to the head and are spaced from the axis of rotation 81 of the shaft and are separated by an angle which is approximately 180°. The spacing determines the depth of the respective braking surface of the inner and outer portions. To prepare the outboard braking surface, the tool is inserted in through the gap 56 within the disc 50. To facilitate insertion of the head 78 in through the gap 56, the machining tool 77 is of suitable shape, such as substantially T-shape as illustrated with the cross piece being arcuate to enable insertion into the gap 56 or in an alternative embodiment (not shown), of a substantially L-shaped configuration. To machine the face, the cutters 80 are located against the disc surface and the tool 77 rotates about the shaft causing the cutters to move against the disc surface. To vary the depth of the cut, relative axial movement between the tool and the disc is provided. The disc 50 is also caused to rotate to provide feed for the tool 77. The relative speed of the rotation of the disc compared to the tool 77 determines the finish on the surface. If the disc rotates more slowly than the tool, a milling process occurs at the surface. If the disc rotates at a faster speed than the tool, then a process similar to a turning process occurs though the rake angle of the cutter would not remain constant as in an ideal turning process due to the rotation of the cutter shaft. The milling process is preferred because the resultant finished texture is of a cris-cross pattern 84 (which is illustrated in Figure 11) which is believed to be better for burnishing the surface than the pattern produced by a turning process, which is similar to a spiral. To prepare the inboard braking surface, a similar tool (not shown) is used except the cutters face outboard rather than inboard as in the tool 77. The inboard surface may be prepared separately or simultaneously with the preparation of the outboard surface. In the latter arrangement, the two machining tools would typically be spaced circumferentially about the disc. An alternative tool 82 is illustrated in Figure 10. This tool is substantially the same as the tool illustrated in Figure 9 except that an additional head 83 is incorporated on a concentric shaft 85 in a position spaced from the head 78. In this arrangement, cutters are located on opposing faces of the heads 78 and 83 and as in the earlier embodiment, the cutters on each head are radially spaced from the axis of rotation of the shaft and separated by an angle which is approximately 180°, though other angles may be used. For example the angle may be 90° which may assist in minimising vibration. The concentric shaft 85 is provided to enable axial displacement between each head and the disc to enable the depth of cut to be varied. With the tool 82 the inboard and outboard braking surfaces can be processed simultaneously thereby enabling the processing time to be further reduced. In other respects the tool 82 processes the faces on the disc in the same way as the earlier embodiment. In yet another alternative, two heads are provided spaced along the axis of the shaft with each head only having a single cutter which is arranged to cut a respective one of the opposing inboard and outboard faces. In this arrangement, each cutter is radially spaced from the axis of rotation of the shaft with the angle between the cutters about the axis of rotation being typically zero such that the cutters are arranged to straddle a particular portion of the disc, or at approximately 180° such that one cutter is arranged to engage the inner portion of the disc while the other cutter engages the outer portion or at any other relative angular position. In an alternative arrangement (not shown) the first portion of the disc may be formed separately from the second portion and may be processed individually, or assembled, and then processed simultaneously. Each of these pieces may be formed in one uniform material or may be composite or incorporate inserted portions. In the manufacture of the disc either as a single piece or in separate pieces, each portion may be solid or ventilated or the disc may be formed with one portion ventilated and the other solid depending on the performance characteristics required. It will be appreciated from the foregoing description that the method of forming a disc in accordance with the invention provides an arrangement which is better able to accommodate the loads involved in operation of the brake assembly while being of light weight construction, being relatively inexpensive and effective in dissipating heat.

Claims: A method of forming a disc (50) for a disc brake assembly (100), the disc being rotatable about an axis of rotation and including a radially outer portion (55) and a radially inner portion (54), each of the outer portion and the inner portion having two opposite sides (52,53) thereof and a gap (56) forming a radial separation between said outer and inner portion and extending in a continuous circle coaxial with the axis of rotation, the method characterised by the steps of: (i) providing a tool (77) including a central shaft (79) and including at least one cutting head (78) having a cutting edge (80) radially spaced from, and rotatable about, the shaft; (ii) locating the cutting tool such that the cutting head is on one side of the disc with the shaft being substantially aligned with said gap; and (iii) rotating the shaft to cause the cutting head to move in a circular path such that the cutting edge engages the one side of both the inner and outer portions of the disc on rotation of the shaft to thereby machine braking surfaces on the one side of both the inner and outer portions of the disc. A method according to claim 1, wherein the shaft (79) of the cutting tool (77) is parallel to said axis of rotation and operable to be located through said gap (56) to enable said cutting tool to machine the other side of said disc (50). A method according to either one of claims 1 or 2, further including the steps of: (i) providing the tool (82) with a second cutting head (83) having a cutting edge radially spaced from the shaft, the second cutting head being spaced from the first cutting (78) head along the shaft; (ii) positioning the cutting tool such that the shaft extends through said gap and that the cutting heads are located on respective opposite sides of said disc; and (iii) rotating the shaft to cause both the cutting heads to move in a circular path such that the cutting edges engage respective ones of the sides of both the inner and outer portions of the disc on rotation of the shaft to thereby simultaneously machine braking surfaces on both sides of both the inner and outer portions of the disc. A method according to any one of the preceding claims, further including the step of changing the relative distance between the disc (50) and at least one of the cutting edges (80) in a direction substantially parallel to the axis of rotation of the shaft (79) to thereby vary the depth of cutting of that cutting edge on machining of the braking surfaces. A method according to any one of the preceding claims, further including the step of providing the tool (77, 82) with the or each cutting head including a plurality of cutting edges (80) angularly spaced around the shaft (79). A method according to any one of the preceding claims, further including the step of rotating the disc (50) about the axis of rotation to provide feed for the cutting tool (77, 82).

Inventors: WANG NUI, WANG, NUI
Applicants: PBR AUSTRALIA PTY LTD
Application year: 1994
Main classification: F16D55
IPC codes: F16D65, F16D69, F16D55